Well tester apparatus

ABSTRACT

A drill stem tester assembly is disclosed including an inner member movable upwardly and downwardly within an outer member, valve means for opening and closing a test passage through said members in spaced longitudinal positions, a balance piston on said inner member having its upper and lower faces subjected respectively to the pressure of fluids within the inner member and in the well annulus, and a bias piston responsive to the hydrostatic head of fluids in the annulus to apply upward force to said inner member.

United States Patent Nutter 51 May 16, 1972 [541 WELL TESTER APPARATUS3,305,023 2/1967 Farley ..166/226 3,308,882 3/1967 Lebourg.. ..l66/l50 Xm] Invent Emmi 381M116 3,494,419 2/1970 Mullins 166/152 x [73] Assignee:Schlumberger Technology Corporation,

New York, NY. Primary Examiner-David H. Brown AttorneyEmest R.Archambeau, Jr., David L. Moseley, Ed- [22] June 1970 ward M. Roney andWilliam R. Sherman [21] Appl. No.: 42,373

[ ABSTRACT 52 US. 01.... ..166 152, 166/226 A stem tester assembly isdimmed including an inner 51 Int. (:1 ..E2lb 49/00 member upwardly anddwmwmdly within 58 Field 6: Search ..166/150 152 226 0.5 member valvemeans for p ng and closing a test passage through said members in spacedlongitudinal positions, a [56] References Cited balance piston on saidinner member having its upper and lower faces subjected respectively tothe pressure of fluids UNlTED STATES PATENTS within the inner member andin the well annulus, and a bias piston responsive to the hydrostatichead of fluids in the annu- 3,384,180 5/1968 Perkins 166/152 x to applyupward force to Said inner 3,249,124 5/1966 Berryman. ....l66/226 X3,358,755 12/1967 Chisholm ..166/152 X 9 Claims, 5 Drawing Figures E 178 :n 45 p 1: 3 M

s a r PATENTEnMAHsmz 3,662,825

' SHEET 1 UP 2 FIG? /0min P. Nutter INVENTOR ATTORNEY PATENTEMHsIm3.662.825

SHEET 2 OF 2 Benjamin P Nutter INVENTOR [wax/ ATTORNEY WELL TESTERAPPARATUS This invention relates generally to a drill stem tester foruse in a well bore, and more specifically to a hydraulically balancedformation tester apparatus employing longitudinally slidable membersthat are manipulated by upward and downward movement of the pipe stringat the surface.

To conduct a drill stem test, a packer and tester valve are lowered intoa well bore on a pipe string and the packer is set to isolate the testinterval. By manipulating the pipe string the tester valve is opened andclosed to flow and shut-in the formations. Pressure recorders make apermanent record of flow and shut-in pressures to enable a subsequentevaluation of the formation. A tester valve that can be operatedpositively and safely, solely in response to upward and downwardmovement of the pipe string, is disclosed in my U.S. Pat. No. 3,308,887,issued Mar. 14, 1967 and assigned to the assignee of the I presentinvention. The tester valve includes a mandrel movable upwardly anddownwardly within a housing, with a mandrel and housing defining a flowor test passage that is opened and closed in spaced longitudinalpositions of the members.

Inasmuch as the valve is operated by longitudinal relative movementbetween parts, it is possible to observe the operation thereof bymonitoring what are called free points" on the rig weight indicator atthe surface. A free point is a weight value on the weight indicator thatremains constant while the pipe string is being moved vertically, andprovides an indication of telescoping motion of a sliding connection inthe test tools. If the pipe string is being lifted, the weight valuewill remain constant until the joint has completely extended, whereuponthe weight value will start to increase. Conversely, if the pipe stringis being lowered, the weight value will remain constant until the jointhas completely closed, whereupon the weight value will begin todecrease. It will readily be appreciated that the tester valve mentionedabove enables the observance at the surface of free points as the valveis cycled, since the structure embodies a longitudinal sliding joint.

It will be recognized, however, that since the test valve is closeduntil the tools are properly positioned so that the pipe string providesa low pressure region with respect to the formations, there is a loss ofbuoyancy the instant the valve is opened. Moreover, the pressures offluids that act on the tester may change during flow and shut-in periodsof the test cycle. In some cases, the pressures act in opposition to theweight of the pipe string and have an influence on the free point valueswhich are monitored at the surface. Particularly where there are othersliding joints in the test tool string, it is desirable to have aconstant free point for the test valve so that a sequence of test tooloperations can be readily recognized at the surface. I 7

It is accordingly a broader object of the present invention to provide anew and improved formation tester assembly that is operated by upwardand downward motion, and is hydraulically balanced in such a manner thatthe loss of buoyancy when the valve is opened does not affect the freepoint. Moreover, for a formation test where liquid is recovered, thefree point for the tester assembly is constant regardless of bottom holeor surface pressure conditions.

This and other objects of thepresent invention are attained through theprovisions of a formation tester apparatus that includes an inner memberthat is sealingly slidable within an outer member between extended andcontracted relative longitudinal positions, the members being adaptedfor connection in a pipe string extending upwardly to the top of thewell bore. The members define a fluid passageway therethrough and havevalve means for opening and closing the passageway in spacedlongitudinal positions. In order to hydraulically balance the members, apiston on the inner member is sealingly slidable within a cylinder inthe outer member with the upper face of the piston being subjected tothe pressure of fluids within the other member, the lower face thereofbeing subject to the pressure of fluids in the well annulus surroundingsaid members. The piston is sized and arranged such that its transversecross-sectional area is substantially equal to transverse areacircumscribed by the seal that prevents fluid leakage between the memberduring telescoping movement. Thus arranged, when buoyancy force is lostas the valve means is opened, this force is supplanted immediately by anupward force on the piston of substantially equal magnitude so that thefree point for the sliding joint embodied in the tester apparatusremains the same and can be readily differentiated at the surface fromthe free points of other sliding joints in the string of test tools.Moreover, the pressures of fluids within the members that act inopposition to the weight of the pipe string are balanced out andcancelled by an equal force acting downwardly on the piston.Accordingly, it will be recognized that the tester assembly ishydraulically balanced so that its free point will remain substantiallyconstant regardless of bottom hole or surface pressure conditions.

The present invention has other objects and advantages that will becomemore readily apparent in connection with the following detaileddescription of the structure and operation of a preferred embodiment,taken in conjunction with the appended drawings in which:

FIG. 1 is a schematic view of drill stern testing equipment to test anoffshore well from a floating vessel;

FIGS. 2A and 2B are longitudinal sectional views of a tester assembly inaccordance with the principle of this invention, FIG. 2B fonning a lowercontinuation of FIG. 2A;

' FIG. 3 is a cross-section taken in line 3-3 of FIG. 2A; and

FIG. 4 is a schematic plan view of the channel system used incontrolling the operating sequence of the tester assembly.

Referring initially to FIG. 1, a schematic representation of anenvironment wherein the present invention has particular utility is inoffshore testing from a floating vessel 10. A well bore 11 intersectsthe particular earth formations to be tested and is usually lined withasteel casing. A wellhead assembly 12 is located at the sea floortogether with a typical blowout preventer stack 13, and a riser 14extends upwardly to the vessel 10. A string of drill stem testing toolsincluding a packer 15 and a tester assembly 16 is connected to the lowerend of a pipe string 17 that extends to the surface and is suspended inthe derrick 18 located on the vessel. The packer 15 can be of the typeshown in U.S. Pat. No. 3,399,729, McGill, dated Sept. 3, 1968 andfunctions to pack off and isolate the interval to be tested. The testerassembly 16 has a valve that is opened and closed by manipulation of thepipe string 17 to flow and shut-in the formations. Typical pressurerecorders 19 make a permanent record of changes in pressure vs. timethat can be subsequently analyzed to determine various characteristicsof the formation. A sample of fluids is produced into the pipe string 17and can be removed for subsequent analysis.

In order to accomodate changes in the total length of the pipe string 17due to the influence of wave and tide action on the vessel 10, topressure and temperature changes that may occur during a test, or tooperation of the test tools, a slip joint 20 is incorporated in the pipestring 17 below the wellhead assembly 12. Also, a second slip joint 21may be disposed between the tester assembly 16 and the packer 15. Thedetails of construction and the slip joints 20 and 21 form no part ofthe present invention and the particular details can be seen, forexample, in U.S. application Ser. No. 42,374, Kisling et al., filedconcurrently herewith and assigned to the assignee of this invention.

The tester assembly 16 in accordance with the principles of thisinvention is shown in FIGS 2A 2D as including an indexing and delaysection 25, a valve and sampler section 26, a hydraulic balancingsection 27 and a hydrostatic bias section 28. The details of theindexing and delay section 25 and the valve and sampler section 26 aredisclosed in U.S. Pat. No. 3,308,887, Nutter, issued Mar. 14, 1967,wherein the mandrel 30 is telescopically disposed within a housing 31for longitudinal movement between extended and contracted positions,relative rotation being prevented by splines 32. A swivel sleeve 33 ismounted on the housing 31 for rotation relative to both the mandrel 30and the housing. The sleeve 33 has an index pin 34 that follows in achannel system 35 that is formed in the exterior of the mandrel. As themandrel 30 is moved longitudinally with respect to the housing 31, theinteraction of the channel system 35 with the index pin 34 causes thesleeve 33 to be indexed between various angular positions with respectto the mandrel 30. In one angular position, inwardly extending lugs 36(FIG. 3) on the sleeve 33 are engaged with companion lugs 37 on themandrel 30 to limit downward movement of the mandrel in an intermediateposition. In the other angular positions, the lugs 36 and 37 bypass oneanother so that the limit of downward movement occurs when the collar 38on the mandrel 30 engages the top of the housing 31, and the limit ofupward movement occurs when the mandrel lugs 37 shoulder against thethickened section 39 of the housing. Referring briefly to FIG. 4, whenthe mandrel 30 is extended the index pin 34 is at position A in thechannel system; as the mandrel is lowered the index pin moves toposition B and the lugs 36 and 37 have bypassed one another; as themandrel is raised the pin moves to position C and the lugs again bypassone another, however, when the mandrel is lowered once again the pinmoves to position D. In this position, the sleeve 33 is located suchthat the lugs 37 on the mandrel 30 engage the sleeve lugs 36 and providean intermediate position stop. Of course again raising the mandrel 30will position the pin 34 in the position A so the cycle is complete. Thepurpose of the indexing system will be explained in detail herebelow inconjunction with the structure and operation of the valve and samplersection 26.

The delay section includes a metering sleeve 41 that works within aclosed hydraulic chamber 42 formed in the housing 31. The meteringsleeve 41 is biased upwardly against a shoulder 43 on the mandrel 30 bya spring 44 and is sized to provide a small clearance between itsoutside surface and the inner wall 45 of the chamber 42 The sleeve 41further has grooves 46 or the like in its inner periphery to providefluid passage spaces. The upper end surface of the metering sleeve 41seats against the shoulder 43 during downward movement of the mandrel 30so that hydraulic fluid contained in the chamber 42 must flow from belowthe sleeve to above it at a metered rate, thus providing a retarding ordelaying action with respect to downward movement. However, the mandrel30 can be moved freely upwardly because the fluid will push the sleeve41 downwardly away from the shoulder 43 and against the action of thespring 44 so that fluid can bypass through the grooves 46 from above thesleeve to below it. The wall surface 47 at the lower section of thechamber 42 can be enlarged in diameter so that the metering actionprovided by the sleeve 41 is released as the mandrel nears the lower endportion of its travel.

The test valve and sampler section 26 of the tester assembly 16comprises longitudinally spaced valve heads 49 and 50 having sealingelements 51 and 52 that engage spaced valve seats 53 and 54 when themandrel 30 is extended or partially contracted, and are free of theseats when the mandrel is fully telescoped within the housing 31. Theannular space 55 in between the valve seats 53 and 54 provides aflow-through sample chamber, in that when the mandrel 30 is in itslowermost position fluids can flow upwardly from ports 56 located belowthe lower valve head 50 through the chamber 55 and through ports 57above a barrier 58 into the bore 59 of the mandrel. However, when themandrel 30 is moved upwardly, the seals 51 and 52 engage the seats 53and 54 simultaneously to trap a sample of fluids in the chamber 55. Itwill be appreciated that the valve is opened only when the mandrel 30 iscompletely telescoped within the housing 31 and that when the mandrel isin a position corresponding to positions C or D in FIG. 3, the valve isclosed to shut-in the formations.

Referring again to FIG. 28, an extension 61 of the mandrel 30 isprovided with an annular piston 62 with the upper face of the pistonexposed to the pressure of fluids within the lower bore 63 of themandrel extension by ports 64. A section 65 of the housing 31 provides acylinder with an inwardly extending flange 66 sealed against the mandrelextension 61 by a seal ring 67, and with the cylinder wall 68 sealedwith respect to the piston 62 by a seal ring 69. The lower face of thepiston 62 is exposed to the pressure of fluids in the annulussurrounding the housing section 65 by ports 71. The transversecross-sectional area of the piston 62 is made substantially equal to thetransverse area encompassed by the outer peripheral surface of themandrel 30. Accordingly, forces due to fluid pressure below and insidethe mandrel 30 tending to elevate it are balanced or cancelled by thesame pressure acting downwardly on the piston 62. The purpose of thisarrangement is to prevent the influence of fluid pressure changes thatoccur during a drill stem test from affecting the longitudinal relativepositions of the mandrel 30 and the housing 31, and to eliminate changesin free point indications at the surface due to such pressure changes.

An additional extension 72 of the mandrel 30 has an annular piston 73that is sealed within a cylinder section 74 of the housing 30 andencloses, together with an inwardly extending shoulder 75, anatmospheric chamber 76 above the piston. The lower face of the piston 73is exposed to the hydrostatic head of fluids in the annulus surroundingthe housing section 74 by ports 77, and of course the upper face of theshoulder 75 is subjected to the same pressures. A seal 78 prevents fluidleakage between the lower end of the mandrel 30 and the housing 31, andother seal rings 79 and 80 prevent leakage into the chamber 76. Thepurpose of this arrangement is to provide a bias force that will tend toelevate the mandrel 30 once it has been lowered. Upon lowering, a forceclue to the difference between the hydrostatic pressure of well fluidsin the annulus and the lesser pressure in the chamber 76 acts in anupward direction on the mandrel 30. Moreover, an identical force actsdownwardly on the shoulder 75 and is transmitted by the housing 30 tothe tools therebelow to aid in retaining a packer seat and the packerbypass closed. The presence of the bias force will ensure that themandrel 30 will move upwardly during a sequence of test toolmanipulations, independently of the movement of other sliding joints inthe system. The lower end of the tester housing 31 has a typicalthreaded pin 81 to adapt it for connection to the tools therebelow.

In operation, the tester assembly 16 and the packer 15 are suspended onthe lower end of the pipe string 17 and lowered into the well bore 11 totest depth. During lowering, the mandrel 30 is extended with respect tothe housing 31 so that the valve assembly is in closed condition withrespect to the flow passage. The pipe string 17 is either empty offluids or can be provided with a water column to cushion the effects offluid entry when the tester is opened. In either event, the bore of thepipe string 17 provides a low pressure region with respect to thepressure of fluids in the formation to be tested, so that formationfluids will flow upwardly and into the pipe string when the tester valveis opened.

By appropriate manipulation of the pipe string, the packer 15 is set toisolate the formation interval to be tested and the weight of the pipestring is applied thereto. As shown in the previously referenced McGillpatent, the packer has a typical bypass passage that is closed as thepacker is set. The weight of the pipe string 17 will cause the mandrel30 of the tester assembly 16 to advance downwardly, such movement beingretarded by the metering action of the metering sleeve 41. The purposeof the metering feature is to ensure that the packer 15 is set and itsbypass is closed before the tester valve has an opportunity to open.

As the mandrel 30 moves downwardly the index pin 34 maintains an angularrelationship of the index sleeve 33 such that the lugs 36 and 37 bypassone another to enable the mandrel to move to its fully contractedposition within the housing 31. When the valve seals 51 and 52 clear theseats 53 and 54, a flow path is provided past the barrier 58 so thatformation fluids can flow upwardly via the ports 56 and 57 and into thepipe string 17. The buoyancy force equal to the hydrostatic head timesthe seal diameter of the mandrel 30 is suddenly lost, however the forceis immediately supplanted by an equal force acting upwardly on thehydraulic balance piston 62. Ac-

cordingly, there is no change in the free point for the tester assembly16. A bias force is also being applied upwardly on the mandrel 30 as thehydrostatic head of fluids acts upwardly on the bias piston 73. Thisforce assists in returning the mandrel 30 upwardly as the testerassembly 16 is cycled and ensures the mandrel will move upwardly toextended position independently of the telescopingmotion of othersliding joints in the system, for example the slip joint 21 locatedbetween the tester assembly 16 and the packer 15.

In order to shut-in the tester assembly to record pressure build-updata, the pipe string 17 is elevated. As the mandrel 30 moves upwardly,the index pin 34 causes the sleeve 33 to swivel to an angular positionsuch that the lugs 36 and 37 again bypass one another and the pin movesto position C, FIG. 4. The valve seals 51 and 52 engage the seats 53 and54 to close off the test passage. The mandrel 30 is then moveddownwardly to its intermediate position when the index pin is at D. Theangular position of the sleeve 33 is such that the lugs 36 and 37 engageone another so that the weight of the pipe string 17 can be imposed onthe housing 31 and the tools therebelow. The valve seals 51 and 52 arestill engaging the seats 53 and 54, however, and the formations areshut-in.

As the pressure of fluids builds up in the isolated zone of the wellbore, the pressure acts upwardly on the seal area of the mandrel 30.However, pressure is also acting downwardly on the balance piston 62,which has an area substantially equal to the seal area of the mandrel.Thus these forces cancel one another and have no effect on thelongitudinal position of the housing 31 or the mandrel 30, and do notaffect the free point indication for the tester at the'surface. The biasforce acting upwardly on the bias piston 73 is constant during the test,since it is dependent only upon the hydrostatic head of the fluids inthe annulus.

To terminate the test, the pipe string 17 is elevated to extend themandrel 30 with respect to the housing 31. Such lifting is continuedafter the free point indications for the tester assembly 16 and thelower slip joint 21 are seen to open the bypass of the packer l5 andequalize pressures. The packer is then unseated and the equipment iswithdrawn from the borehole.

It will be appreciated that the tester assembly is hydraulicallybalanced so that it will have a constant free point during the verticalpipe manipulations that are applied at the surface. The free point ofthe tester is thus readily differentiated from the free points of othersliding joints in the system, to make surface control of the tools morepositive and reliable. Since certain changes or modifications may bemade in the disclosed embodiment without departing from the inventiveconcepts involved, the aim of the appended claims is to cover all suchchanges and modifications falling within the true spirit and scope ofthe present invention.

lclaim:

l. A well testing apparatus comprising: an inner member movable upwardlyand downwardly between spaced longitudinal positions within an outermember, said members defining a fluid passageway, said passageway beingprovided in part by upper and lower bores in said inner member extendingrespectively above and below a transverse barrier section disposedintermediate the ends of said inner'member and in part by a flow pathextending externally of said inner member between locations above andbelow said barrier section seal means for preventing fluid leakagebetween said members during upward and downward movement; valve means onsaid members responsive to upward and downward movement of said innermember for opening and closing said second mentioned part of said flowpassage; and piston means on said inner member below said barriersection sealingly slidable within cylinder means in said outer member,the upper face of said piston means being subject at all relativepositions of said inner and outer members to the pressure of fluids insaid passageway below said valve means the lower face of said pistonmeans being subject at all relative positions of said inner and outermembers to the pressure of fluids in the well annulus surrounding saidmembers, the transverse cross-sectional area of said piston means beingsubstantially equal to the transverse area circumscribed by said sealmeans so that the pressures of fluids within said passageway below saidvalve means act with substantially equal force in opposite longitudinaldirections and said inner member is balanced with respect to suchpressures I 2. The well testing apparatus of claim 1 wherein saidcylinder means is provided by an annular chamber formed between spacedshoulder sections of said outer member that are sealed with respect toequal diameter portions of said inner member, the respective faces ofsaid piston means being subjected to fluid pressure by ports through thewall of said inner member above said piston means and through the wallof said cylinder means adjacent to the lower one of said spaced shouldersections.

3. The well testing apparatus of claim 2 wherein said valve meansincludes axially spaced sealing elements that simultaneously engageaxially spaced valve seat surfaces when the valve means is in closedposition to trap a sample of fluids flowing through said passageway.

4. The well testing apparatus of claim 2 further including spline meansfor preventing relative rotation of said members during upward anddownward movement.

5. A well testing apparatus comprising: an inner member movable upwardlyand downwardly within an outer member, said members defining a fluidpassageway;

seal means for preventing fluid leakage between said members;

valve means on said members for opening and closing said passageway inspaced longitudinal relative positions of said members;

first piston means on said inner member sealingly slidable within firstcylinder means in said outer member, the upper face of said piston meansbeing subject to the pressure of fluids in said passageway, the lowerface of said piston being subject to the pressure of fluids in the wellannulus surrounding said members, the transverse crosssectional area ofsaid piston means being substantially equal to the transverse areacircumscribed by said seal means; and

second piston means on said inner member sealingly slidable withinsecond cylinder means in said outer member and enclosing a low pressurechamber above said second piston means, the lower face of said pistonmeans being subject to the hydrostatic head of fluids in the well boreto provide an upwardly directed bias force on said mandrel.

6. The well tester apparatus of claim 5 wherein said inner member has abarrier to block the through-bore thereof, said passageway extending inpart externally of said inner member and between axially spaced lateralports through the wall of said inner member, said valve means includingvalve elements disposed on said inner member between said axially spacedports, the lowermost one of said ports being located above said firstpiston means.

7. The well packer apparatus of claim 6 wherein said valve elementscomprise axially spaced valve heads having sealing elements engageablewith spaced valve seats on said outer member, the annular cavitydisposed between said inner and outer members and said valve seatsproviding a flow-through sample chamber in which a sample of fluids istrapped when said valve heads engage said seats.

8. A well tester apparatus comprising: a mandrel movable upwardly anddownwardly within a housing, the upper end of said mandrel and the lowerend of said housing being adapted for connection to a pipe string, saidmembers defining a fluid passageway; seal means for preventing fluidleakage between said mandrel and said housing; valve means on saidmandrel and said housing for closing said fluid passageway in an upperposition of said mandrel with respect to said housing and for openingsaid fluid passageway in a lower position of said mandrel with respectto said housing; a first piston section on said mandrel below said valvemeans and sealingly slidable within a first cylinder means on saidhousing, the upper and lower transverse surfaces of said piston sectionbeing exposed respectively to the pressure of fluids inside said mandreland to the pressure of fluids in the well bore externally of saidhousing, the effective pressure area of said piston section besubstantially equal to the transverse area circumscribed by said sealmeans; and a second piston section on said mandrel below said firstpiston section and sealingly slidable within a second cylinder means onsaid housing, said mandrel, second piston section and said secondcylinder means enclosing a low pressure chamber above said second pistonsection, the lower transverse surface of said second piston sectionbeing exposed to the pressure of fluids in the well annulus surroundingsaid housing.

9. The well tester apparatus of claim 8 wherein said flow passagewayextends in part through an annular cavity formed between said mandreland said housing and between axially spaced ports through the wall ofsaid mandrel, said seal means being positioned between said mandrel andhousing above the uppermost one of said ports, said first piston sectionbeing located on the mandrel below the lowermost one of said ports.

* i i i i

1. A well testing apparatus comprising: an inner member movable upwardlyand downwardly between spaced longitudinal positions within an outermember, said members defining a fluid passageway, said passageway beingprovided in part by upper and lower bores in said inner member extendingrespectively above and below a transverse barrier section disposedintermediate the ends of said inner member and in part by a flow pathextending externally of said inner member between locations above andbelow said barrier section ; seal means for preventing fluid leakagebetween said members during upward and downward movement; valve means onsaid members responsive to upward and downward movement of said innermember for opening and closing said second mentioned part of said flowpassage; and piston means on said inner member below said barriersection sealingly slidable within cylinder means in said outer member,the upper face of said piston means being subject at all relativepositions of said inner and outer members to the pressure of fluids insaid passageway below said valve means , the lower face of said pistonmeans being subject at all relative positions of said inner and outermembers to the pressure of fluids in the well annulus surrounding saidmembers, the transverse cross-sectional area of said piston means beingsubstantially equal to the transverse area circumscribed by said sealmeans so that the pressures of fluids within said passageway below saidvalve means act with substantially equal force in opposite longitudinaldirections and said inner member is balanced with respect to suchpressures .
 2. The well testing apparatus of claim 1 wherein saidcylinder means is provided by an annular chamber formed between spacedshoulder sections of said outer member that are sealed with respect toequal diameter portions of said inner member, the respective faces ofsaid piston means being subjected to fluid pressure by ports through thewall of said inner member above said piston means and through the wallof said cylinder means adjacent to the lower one of said spaced shouldersections.
 3. The well testing apparatus of claim 2 wherein said valvemeans includes axially spaced sealing elements that simultaneouslyengage axially spaced valve seat surfaces when the valve means is inclosed position to trap a sample of fluids flowing through saidpassageWay.
 4. The well testing apparatus of claim 2 further includingspline means for preventing relative rotation of said members duringupward and downward movement.
 5. A well testing apparatus comprising: aninner member movable upwardly and downwardly within an outer member,said members defining a fluid passageway; seal means for preventingfluid leakage between said members; valve means on said members foropening and closing said passageway in spaced longitudinal relativepositions of said members; first piston means on said inner membersealingly slidable within first cylinder means in said outer member, theupper face of said piston means being subject to the pressure of fluidsin said passageway, the lower face of said piston being subject to thepressure of fluids in the well annulus surrounding said members, thetransverse cross-sectional area of said piston means being substantiallyequal to the transverse area circumscribed by said seal means; andsecond piston means on said inner member sealingly slidable withinsecond cylinder means in said outer member and enclosing a low pressurechamber above said second piston means, the lower face of said pistonmeans being subject to the hydrostatic head of fluids in the well boreto provide an upwardly directed bias force on said mandrel.
 6. The welltester apparatus of claim 5 wherein said inner member has a barrier toblock the through-bore thereof, said passageway extending in partexternally of said inner member and between axially spaced lateral portsthrough the wall of said inner member, said valve means including valveelements disposed on said inner member between said axially spacedports, the lowermost one of said ports being located above said firstpiston means.
 7. The well packer apparatus of claim 6 wherein said valveelements comprise axially spaced valve heads having sealing elementsengageable with spaced valve seats on said outer member, the annularcavity disposed between said inner and outer members and said valveseats providing a flow-through sample chamber in which a sample offluids is trapped when said valve heads engage said seats.
 8. A welltester apparatus comprising: a mandrel movable upwardly and downwardlywithin a housing, the upper end of said mandrel and the lower end ofsaid housing being adapted for connection to a pipe string, said membersdefining a fluid passageway; seal means for preventing fluid leakagebetween said mandrel and said housing; valve means on said mandrel andsaid housing for closing said fluid passageway in an upper position ofsaid mandrel with respect to said housing and for opening said fluidpassageway in a lower position of said mandrel with respect to saidhousing; a first piston section on said mandrel below said valve meansand sealingly slidable within a first cylinder means on said housing,the upper and lower transverse surfaces of said piston section beingexposed respectively to the pressure of fluids inside said mandrel andto the pressure of fluids in the well bore externally of said housing,the effective pressure area of said piston section be substantiallyequal to the transverse area circumscribed by said seal means; and asecond piston section on said mandrel below said first piston sectionand sealingly slidable within a second cylinder means on said housing,said mandrel, second piston section and said second cylinder meansenclosing a low pressure chamber above said second piston section, thelower transverse surface of said second piston section being exposed tothe pressure of fluids in the well annulus surrounding said housing. 9.The well tester apparatus of claim 8 wherein said flow passagewayextends in part through an annular cavity formed between said mandreland said housing and between axially spaced ports through the wall ofsaid mandrel, said seal means being positioned between said mandrel andhousing above the uppermost one of said ports, said first piston sectionbeing located on the maNdrel below the lowermost one of said ports.